Load-bearing structures

ABSTRACT

In one aspect, the present invention provides a load-bearing structure with a novel interlocking edge. Also provided is a method of making a load-bearing structure having an overlapping edge, in which structural members are combined so that a stepped-down lip of one structural member covers a peripheral edge of another member. Also provided is a load-bearing structure having cellular structure and which is useful with or without an overlapping edge. In addition, there is provided a molding method for making cellular, load-bearing structures.

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/475,319, filed on Jun. 5, 1995, now U.S. Pat. No. 5,607,531,which issued on Mar. 4, 1997.

TECHNICAL FIELD

This invention relates to load-bearing structures, to the use ofreinforcing fiber/polymer composites in load-bearing structures, and toload-bearing structures for making a load-bearing surface.

BACKGROUND ART

Load-bearing structures with overlapping edges which are fitted togetheror otherwise combined to make a load-bearing surface, are well known.Illustrative are hard wood flooring pieces provided with tongue andgroove interfit. Nevertheless, there is a need for a load-bearingstructure having an improved interlocking edge, and in particular withreduced weight but yet good load-bearing properties.

Thermoplastic polymer/reinforcing fiber composites are used to replacemetals in aerospace and automotive applications. However, newapplications have been slow due to the higher cost of composites whencompared to conventional materials like metal, wood or concrete. If thecost could be adequately reduced and yet the strength could bemaintained, numerous applications exist where thermoplasticpolymer/reinforcing fiber composites could replace metal, wood orconcrete. These applications include furniture framing, power poles,shipping containers, crates, shipping pallets, platforms, constructiondecking, bridge decking, mud pads, road mats, driveways, warehouseflooring, and flooring for trucks and trailers.

Known uses of thermoplastic polymer/reinforcing fiber composites furtherinclude reinforcing bars or re-bars, reinforced mats and reinforcedfabrics. These uses are illustrated in U.S. Pat. No. 4,154,634 toShobert et al, U.S. Pat. No. 4,752,513 to Rau et al, and U.S. Pat. No.5,190,809 to Marissen et al. Advantageously, the thermoplastic polymerstrengthens and stiffens the composite. This type of polymer is oftentermed a "matrix resin" in the context of polymer/reinforcing fibercomposites.

Engineering type, plastic resins such as polyester, nylon, polyphenylenesulfide and polyurethane are frequently used as matrix resins infiberglass composite laminates and pultrusion profiles. When solid,these resins are very rigid and provide the most stiffness, but fracturewhen overstressed, often leaving sharp splintered edges.

Accordingly, there is a need for load-bearing structures of reduced costand yet good load-bearing properties. In addition, if fiber/matrix resincomposites are used in certain load-bearing structures, there is a needfor flexibility of the composites so that fracturing and sharpsplintered edges do not result.

SUMMARY OF THE INVENTION

In accordance with the present invention, an improved load-bearingstructure with an overlapping edge to be fitted together with like partsto make a load-bearing surface, is provided. Beneficially, the structureincludes an edge which interlocks with an edge of a like part to preventslippage or movement in a direction parallel to the interlocking edges.This load-bearing structure is advantageously provided with cellularstructure for weight and cost reduction. Preferably, for strength, thecellular structure is a honeycomb structure.

In accordance with the invention, a method of manufacturing aload-bearing structure having an overlapping edge, includes making andassembling a pair of structural members each including interiorlylocated structure defining a first plane, a peripheral edge having asurface defining a second plane which extends from the first plane, anda stepped-down lip having a surface defining a third plane which extendsfrom the first plane, wherein the second and third planes are oppositelyoffset from the first plane. By "interiorly located" is meant locatedinterior to a peripheral edge or stepped-down lip, that is, not in aperipheral edge or stepped down lip. In accordance with the method,these structural members are combined so that a stepped-down lip of eachstructural member covers a peripheral edge of the other structuralmember.

Beneficially, the assembled structural members are visually identicalmembers. However, the method is also applicable to structural membersthat are not visually identical. Advantageously, the structural membersmay be molded.

A key aspect is the use of a stepped-down lip from one member to cover aperipheral edge of the other member. As a result, an edge is made thatis given strength by both structural members. Advantageously, thestepped-down lip may also cover exposed cell structure in a peripheraledge. Further edge strengthening may be achieved using a stepped-downlip having a solid cross-section, that is, no cellular structure.

Also in accordance with the present invention, is a molded, load-bearingstructure having cellular structure and yet good load-bearingproperties, and which is useful apart from or in combination with anoverlapping edge such as the previously described, interlocking edge. Inthe latter case, a solid cross-section, stepped-down lip beneficiallyincludes reinforcing fiber/matrix resin composite. This assists thelower half of an overlap which must carry the full load along the edgeas the load moves from one section of a load-bearing surface to thenext.

In one aspect, this load-bearing structure beneficially includes skinstructure adjacent to and covering cellular structure, and in particularcellular structure sandwiched between skin structures. Reinforcingfiber/matrix resin composite may be advantageously located in the skinstructure, as well as in the cellular structure.

In accordance with the present invention, reinforcing fiber/matrix resincomposite is thermally bonded to adjacent thermoplastic structure. Inthis regard, it has been found that the compression strength, stiffnessand stress rupture of large relatively flat plastic articles arebenefitted by molding reinforcing fiber/matrix resin composites into anarticle during manufacture, and using a matrix resin compatible with themolding resin so as to result in thermally bonding or fusing togetherinto a solid mass.

Advantageously, when the fiber/matrix resin composite is located in thecellular structure, the composite is disposed within walls forming thecellular structure. When the fiber/matrix resin composite is locatedexterior to the cellular structure, it has been discovered that alayered skin structure including fibrous reinforcing structure in theform of fibrous web structure provides advantageous load-bearingproperties.

In certain applications, a cellular thermoplastic, load-bearingstructure may be at risk of floating away, if flooded. To avoid this,walls forming the cellular structure may include bleeder holesconnecting the cells, and the cells may be filled with a fluid. Watermay be used as the fluid provided that the structure has a specificgravity greater than one. If water or a like aqueous fluid is used tofill the cells, a gelling agent may be used in an effective amount.

In another aspect, the cellular, load-bearing structure includes aplurality of reinforcing fiber/matrix resin composite structures,disposed within walls forming the cellular structure. In an embodimentof this structure, cells are open to the ambient environment, that is,not sandwiched between and covered by skin structures. Beneficially,composite structure is thermally bonded to adjacent thermoplasticstructure.

In applications where some flexibility is desired and fracturing andsharp edges are not acceptable, polyolefins such as polyethylene andpolypropylene, may be advantageously used as the matrix resin offiber/matrix resin composite.

Also provided is a method of molding. By the method, there are placedinto a lower mold member in sequence, a first thermoplastic polymer,reinforcing fiber/matrix resin composite, and a second thermoplasticpolymer, the matrix resin being compatible with the first and secondthermoplastic polymers. In addition, a plurality of fibrous reinforcingmembers are vertically disposed within wall-forming portions of an uppermold member. Then, the lower and upper mold members are combined undersuitable molding conditions, and after cooling, there is demolded acomposite structure comprising a layer of the first thermoplasticpolymer fusion bonded to the reinforcing fiber/matrix resin compositefusion bonded to a layer integral with cell-forming walls and formed ofthe second thermoplastic polymer, the fibrous reinforcing members beingmolded into the cell-forming walls. Advantageously, the fibrousreinforcing members are fusion bonded to the adjacent wall structure.

In the drawing and in the detailed description of the invention thatfollows, there are shown and essentially described only preferredembodiments of this invention, simply by way of illustration of the bestmode contemplated of carrying out this invention. As will be realized,this invention is capable of other and different embodiments, and itsseveral details are capable of modification in various respects, allwithout departing from the invention. Accordingly, the drawing and thedetailed description are to be regarded as illustrative in nature, andnot as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying drawing, which forms a part ofthe specification of the present invention, and which depicts preferredembodiments in accordance with the present invention.

FIG. 1 is a perspective view of a preferred load-bearing structure inaccordance with the present invention;

FIG. 2 is a plan view of a structural member of the load-bearingstructure of FIG. 1;

FIG. 3 is a cross-sectional view taken substantially along line 3--3 ofFIG. 2;

FIG. 4 is a cross-sectional view taken substantially along line 4--4 ofFIG. 2;

FIG. 5 is a cross-sectional view taken substantially along line 5--5 ofFIG. 2;

FIG. 6 is an assembly view showing relative orientation of structuralmembers for making the load-bearing structure of FIG. 1;

FIG. 7 is a perspective view of a prior art load-bearing structure;

FIG. 8 is a plan view of a structural member for making a secondembodiment of a preferred loading-bearing structure in accordance withthe present invention;

FIG. 9 is a cross-sectional view taken substantially along line 9--9 ofFIG. 8;

FIG. 10 is a cross-sectional view taken substantially along line 10--10of FIG. 8;

FIG. 11 is a plan view of a structural member which is the mirror imageof the structural member of FIG. 8;

FIG. 12 is a cross-sectional view taken substantially along line 12--12of FIG. 11;

FIG. 13 is an assembly view showing relative orientation of thestructural members of FIGS. 8 and 11, for making the second embodimentof a load-bearing structure in accordance with the present invention;

FIG. 14 is a partial cross-sectional view showing internal details of apreferred cellular, load-bearing structure in accordance with thepresent invention;

FIG. 15 is a partial cross-sectional view taken substantiallyperpendicular to the view of FIG. 14;

FIG. 16 is a partial cross-sectional view of a stepped-down lip inaccordance with the present invention; and

FIG. 17 is a partial cross-sectional view of another embodiment of acellular, load-bearing structure in accordance with the presentinvention.

DETAILED DESCRIPTION

As mentioned, in one aspect, the present invention relates to improvedload-bearing structures with overlapping edges to be fitted together tomake a load-bearing surface such as flooring, driveways and decking, ora temporary surface. Referring to FIGS. 1 to 6, a preferred load-bearingstructure 10 in accordance with the present invention, is provided.

With particular reference to FIG. 1, load-bearing structure 10 includesan upper stratum 12 which overhangs a lower stratum 14. Moreparticularly, generally rectangular structure 10 includes opposing upperand lower offsets 16,17 located along the width, and opposing upper andlower offsets 18,19 and opposing upper and lower offsets 20,21 locatedalong the length.

An important feature is that upper and lower offsets are disposed in aside-by-side relationship. Thus, upper offset 18 and lower offset 21 ofone length are located side-by-side, as well as upper offset 20 andlower offset 19 of the other length. Accordingly, structure 10 includessplit overlaps along the length, but a single overlap along each width.As a result of the split overlaps, an adjacent, like configured,structural member situated to mate with offsets 18,21 or offsets 19,20,interlocks with structure 10. Beneficially, the interlocking preventsslippage or movement of the interlocked structures in the lengthwisedirection. More particularly, ends 22,24 of offsets of different strata,cooperate to oppose movement in the lengthwise direction.

As will be understood, the width could be configured with side-by-sideupper and lower offsets. Also, it is not necessary that the structure berectangular or be limited to four sides. For example, the structurecould be square or could have six or more sides such as a hexagon or anoctagon.

Load-bearing structure 10 may be formed of a variety of well knownmaterials including wood, plastic and rubber. However, a limitation isthat the material be suitable for a load-bearing surface.

With particular reference to FIGS. 2 to 6, structure 10 beneficially isprovided with cellular structure for weight and cost reduction,including cellular structure in the offsets. Advantageously, thecellular structure is a honeycomb structure.

In accordance with the present invention, structure 10 is advantageouslyassembled from molded structural members. Referring particularly toFIGS. 3 to 6, a molded structural member 30 includes interiorly located,exposed cell structure 32 defining a first plane A; exposed cellstructure 34 in a peripheral edge 36 having a surface defining a secondplane B which extends from plane A; and a stepped-down lip 40 having asurface 38 which defines a third plane C, which also extends from planeA. As indicated, planes B and C are beneficially oppositely offset fromplane A by a substantially equal distance. The stepped-down lip isbeneficially solid in cross-section, that is, does not have cellularstructure.

As further shown, advantageously planes A,B are connected by chamferedtransition surfaces 42, and planes A,C are connected by chamferedtransition surfaces 44; in other words, the transition surfaces are notat right angle to, but rather are disposed oblique to, the connectedplanes. A radius 45 connects stepped-down lip 40 and a side wall 46 ofthe member, and an end 47 of the stepped-down lip is likewise defined bya radius.

With reference to FIG. 6, in accordance with the present invention,structure 10 may be advantageously assembled from two visuallyidentical, structural members. As a result, only a single mold isnecessary to make both structural members, and this provides costsavings. FIG. 6 depicts the relative orientation of structural members30,30 for assembly, with one member rotated 180 degrees relative to theother member. As shown, structural members 30,30 are combined so thatstepped-down lips 40 of each structural member cover cellular peripheraledges 36 of the other structural member.

Referring again to FIG. 1, it may be seen that an offset is verticallyformed by a stepped-down lip 40 and a peripheral edge 36. Moreparticularly, for both split overlaps, a pair of stepped-down lips coverand reinforce a pair of cellular peripheral edges. Note particularlythat offsets 16,17,18,20 and 21 are so depicted in FIG. 1. As a result,an offset or edge is strengthened by both structural members.

Referring to FIG. 7, a prior art load-bearing structure 50 to becombined with like parts for making a load-bearing surface, is shown.Structure 50 is formed by offsetting solid layers so that an upper layer52 overhangs a lower layer 54 on two of its sides. As a result, thestructure includes opposing upper and lower offsets 55,56 and opposingupper and lower offsets 57,58, which provide an edge for overlappingwith like structures in making a load-bearing surface.

In accordance with the present invention and with reference to FIGS. 8to 13, structure 50 beneficially is modified to have a cellularstructure, including cellular structure in the offsets, and a modifiedstructure 100 is advantageously assembled from molded structural membersas described. FIGS. 8 to 10 show one structural member 60, and FIGS. 11and 12 show a mirror image, structural member 80.

Referring to FIGS. 8 to 10, and in particular to FIGS. 9 and 10, amolded structural member 60 includes interiorly located, exposed cellstructure 62 defining a first plane A'; exposed cell structure 64 in aperipheral edge 66 having a surface defining a second plane B' whichextends from plane A'; and a stepped-down lip 70 having a surface 68which defines a third plane C', which also extends from plane A'. Asindicated, planes B' and C' are beneficially oppositely offset fromplane A' by a substantially equal distance.

As further shown, advantageously planes A',B' are connected by chamferedtransition surfaces 72, and planes A',C' are connected by chamferedtransition surfaces 74. Thus, transition surfaces 72,74 are disposedoblique to the connected planes. A radius 75 connects stepped-down lip70 and a side wall 76 of the member, and an end 77 of the stepped-downlip is likewise defined by a radius.

Referring to FIGS. 11 and 12, and in particular to FIG. 12, mirrorimage, structural member 80 includes interiorly located, exposed cellstructure 82 defining a first plane A"; exposed cell structure 84 in aperipheral edge 86 having a surface defining a second plane B" whichextends from plane A"; and a stepped-down lip 90 having a surface 88defining a third plane C", which also extends from plane A". Planes B"and C" are beneficially oppositely offset from plane A" by asubstantially equal distance.

As further shown, advantageously planes A", B" are connected bychamfered transition surfaces 92, and planes A", C" are connected bychambered transition surfaces 94. Thus, the transition surfaces aredisposed oblique to the connected planes. A radius 95 connectsstepped-down lip 90 and a side wall 96 of the member, and an end 97 ofthe stepped-down lip is likewise defined by a radius.

With reference to FIG. 13, in accordance with the present invention,structure 100 may be assembled from structural members 60,80. FIG. 13depicts the relative orientation of these structural members forassembly. As shown, structural members 60,80 are similarly combined asin FIG. 6 so that stepped-down lips of each structural member coverperipheral edges of the other structural member. In this case, astepped-down lip covers and reinforces a cellular peripheral edge on allfour sides.

As stated, a key aspect is the use of a stepped-down lip from one memberto cover a peripheral edge of the other member. As a result, an offsetor edge is strengthened by both structural members. This advantagebroadly applies to overlapping edges in which load-bearing propertiesare of importance.

In accordance with the present invention, a molded, load-bearingstructure having cellular structure and good load-bearing properties, isprovided. Referring to FIGS. 14 and 15, a preferred molded structure 110includes a cellular structure formed by walls 114, sandwiched betweenmultilayer skin structures 116,118. This structure is useful incombination with overlapping edges, in which case FIGS. 14 and 15 may beregarded as partial cross-sectional views taken from FIG. 1 showingadditional details. Without overlapping edges, this structure is usefulfor furniture framing, power poles, containers, crates, shipping palletsand so forth. The cellular structure is, as shown, preferably ahoneycomb structure.

Inner layers 120,122 of skins 116,118 are advantageously integral withcell-forming walls 114. To this end, walls 114 and inner layers 120,122are beneficially made from the same thermoplastic material.

Adjacent to inner layers 120,122 are reinforcing fiber layers 124,126comprised of reinforcing fiber 128 coated with a suitable matrix resin130. The fiber is beneficially a fibrous structure such as a woven webdisposed or encapsulated within the matrix resin. Alternative usefulfibrous web structures are likewise advantageously open so as to permitmatrix resin flow through the web, and include nonwoven andneedle-punched webs. Exemplary useful fiberglass woven fabric iscommercially available from Dynatron/Bondo and has a thickness of 0.010inches.

Beneficially disposed exterior to and adjacent the reinforcing fiberlayers are thermoplastic layers 132,134. In accordance with theinvention, to enhance load-bearing properties, reinforcing fiber layer124 is bonded to inner layer 120 and exterior layer 132, and likewisereinforcing fiber layer 126 is bonded to inner layer 122 and exteriorlayer 134. It is especially advantageous for a fiber layer to becoextensive with the respective inner and outer layer because amongother things, greater surface area for uniting the layers is therebyprovided. Layer-to-layer bonding is provided for by reinforcing fiberbeing thoroughly coated with, and encapsulated or disposed within, thematrix resin. Inadequate bonding would result in fibrous layerdetachment and structural failure under load.

To provide for bonding between a reinforcing fiber layer and an adjacentthermoplastic layer, the matrix resin of a fiber layer is selected to becompatible with thermoplastic polymers of adjacent layers. To this end,the thermoplastic matrix resin may be the same as or different thanthermoplastic polymers of adjacent layers, but in any event thematerials are beneficially compatible so as to thermally bond to or fusewith or to one another, under heat and pressure. It is well known thatcertain thermoplastics are incompatible with one another so thatdelamination of or separation between adjacent layers occurs; therefore,disposing incompatible thermoplastics in contacting layers will beadvantageously avoided when assembling a structure in accordance withthe present invention. Within this context, a variety of thermoplasticpolymers may be used as the matrix resin and adjacent layer polymers.

Exemplary suitable thermoplastic polymers include polyolefins such aspolyethylene and polypropylene, polyesters such as polyethyleneterephthalate, polymers of chlorinated ethylene monomers such as vinylchloride, polyphenylene sulfide, and liquid crystalline polymers, inparticular melt-processable liquid crystalline polymers. However, inload-bearing applications where some flexibility is desired andfracturing and sharp edges are not acceptable, polyolefins such aspolyethylene and polypropylene advantageously are beneficially used asthe matrix resin, whereas polyesters and polyphenylene sulfide would beavoided. Otherwise, engineering type plastic resins are preferred as thematrix resin, for rigidity and stiffness.

For traction in the case of, for instance a road surface, beneficiallydisposed exterior to thermoplastic layers 132,134 are traction-enhancinglayers 136,138 made of, for example rubber. Layers 136,138 arebeneficially bonded to underlying layers 132,134. Provision of layers136,138 as both exterior surfaces of structure 100 makes either exteriorsurface usable as a gripping road surface. If desired, one or both oflayers 136,138 may be omitted without effect upon load-bearingproperties, compared to these properties when a material such as rubberis used for traction. Similarly, an exterior layer or layers may beprovided for another purpose.

Referring again to reinforcing fiber layers 124, 126, each layer isadvantageously a composite structure containing reinforcing fiber in theform of a pair of fibrous web structures 128. In accordance with theinvention, these layers may be beneficially made using a high meltindex, high density polymer as the matrix resin. Thus, a usefulpolyethylene matrix resin will have a melt index of from about 15 to 30gm/10 min, and a density in the range of about 0.95 to 0.965 gm/cc,preferably a melt index of about 20 to 25 gm/10 min, and a density ofabout 0.95 to 0.96 gm/cc.

When using a high melt index, high density polymer and a pair of fibrousweb structures to make a reinforcing fiber layer, it is advantageous tobuild a multilayer structure of polymer film, fibrous web structure,polymer film, fibrous web structure, and polymer film, and thereafter toconsolidate by applying suitable heat and pressure to the multilayerstructure. Under heat and pressure, a high melt index, high densitypolymer advantageously flows through a fibrous web structure and lockswith like polymer on the other side to provide a unitary structure thatresists delamination. Polymer film between fibrous web structuresprevents blocking; otherwise, these web structures may be pulled apartfrom one another after consolidation. An example of a suitable high meltindex, high density matrix resin has a melt index of about 23 gm/10 min,and a density of about 0.953 gm/cc, and is commercially available asExxon 6705.

A low melt index polymer such as a linear low density polyethylene(LLDPE), may be used as a matrix resin, under conditions of highpressure provided by for instance, a hydraulic press. However, if aninadequate consolidating pressure is used to force flow of a low meltindex polymer, reinforcing fiber layers 124,126 may delaminate orseparate.

A useful LLDPE polymer will be a copolymer of ethylene and minor amountsof alkenes having beneficially from about 3 to 8 carbons per alkenemolecule. A particularly useful alkene is hexene, in particular1-hexene. The LLDPE polymer will typically have a density in the rangeof about 0.920 to 0.930 gm/cc, and a melt index in the range of about 1to 4 gm/10 min, preferably about 1.5 to 2.5 gm/10 min. An exemplarysuitable LLDPE polymer is an ethylene-hexene copolymer having a densityof 0.918 gm/cc and a melt index of 2 gm/10 min, and available as Novacor0218.

Although a reinforcing fiber layer may consist of a single fibrous webstructure and matrix resin, a pair of spaced apart, fibrous webstructures in a reinforcing fiber layer, beneficially provide increasedstructural strength. If desired, a reinforcing fiber layer may includeadditional spaced apart, fibrous web structures.

Alternatives to reinforcing fiber layers based upon fibrous webstructures, include reinforcing fiber layers formed by spaced apart,reinforcing fiber/matrix resin bars or re-bars. To make thesereinforcing composite structures, filaments of fiber may be coated withmolten matrix resin using filament coating technology. Especially usefulfilament coating technology is described in U.S. Pat. No. 5,607,531,which issued on Mar. 4, 1997, the pertinent disclosure of which ishereby incorporated, herein by reference. Filaments may also be coatedwith powder and squeezed with heated rolls similar to roving. In anycase, filaments are beneficially coated with a matrix resin and theindividually coated filaments disposed or encapsulated within the matrixresin. These composite structures are likewise advantageously disposedwithin, rather than exterior to, the skin structure.

A typical thickness for useful re-bar will be on the order of about0.080 to 0.200", although the thickness may vary. These bars may be in avariety of shapes and may be oriented in a variety of patterns. However,flat or round bars are most common, and a lattice is a useful pattern.

In accordance with the invention, and with continued reference to FIGS.14 and 15, composite structure 110 may be further stiffened byreinforcing members 140 beneficially vertically disposed withincell-forming, vertical walls 114. By comparison, the cellular structuremay be weakened if cellular wall structure is disrupted by locatingfibrous reinforcing members outside walls 114.

Similar to a reinforcing fiber layer, fibrous reinforcing members 140are advantageously comprised of reinforcing fiber 142 disposed orencapsulated within a suitable matrix resin 144 so as to provide forthermal bonding or fusion to adjacent thermoplastic structure. A usefulthickness of these members is about one-half the total wall thickness,for instance, 3/32" for a total wall thickness of about 3/16". As may beunderstood, relatively flat re-bar of greater width than thickness maybe formed by pultrusion using a shaping die of appropriate apertureshape and size, and vertically disposed as shown. As in the case ofre-bar in the skin structures, the number of members 140 to be used, thespacing between members, and the orientation pattern of members willdepend upon the load-bearing properties desired.

Referring to FIG. 17, a related useful cellular, load-bearing structure190 similar to that of U.S. Pat. No. 5,405,567, is shown. As describedtherein, the shaped hollows 192 of structure 190 may be a variety ofshapes such as square, hexagon, octagon and so forth. A distinguishingfeature is the strengthening presence of reinforcing fiber/matrix resinre-bars 194 in cell-forming, vertical walls 196. Beneficially, re-bars194 are thermally bonded to adjacent wall structure.

With reference to Table 1, structure 190 including re-bars in thecell-forming walls (indicated as Single Ply with re-bars), is comparedrelative to deflection and break, with different woods. The diameter ofthe rebar is 1/2" and the thickness of the structure is 1". Alsocompared in Table 1 is a multilayer load-bearing structure (indicated asThree Ply with re-bars) made by welding together a stack of threestructures each corresponding to structure 190, with the centerstructure being oriented so that the rebar thereof is perpendicular tothe direction of the rebar of the top and bottom structures. Thedeflection is the force in PSI required to produce a one inch deflectionacross a span of 11 1/2". Also shown in the Table is whether or not thematerial broke under the load.

                  TABLE 1    ______________________________________              DEFLECTION            WEIGHT    MATERIAL  PSI/INCH      BREAK   1 × 12 × 12"    ______________________________________    Rough Oak 12,500        No      13.3#    Rough Ash 10,500        Yes     11.6    Rough Gum 11,000        Yes     8.8    Dried Oak 18,800        Yes     --    Dried Ash 13,400        Yes     --    Green Oak 11,900        Yes     --    Green Ash 11,200        Yes     --    Green Gum  9,600        Yes     --    Single Ply               5,100        No      4.4    With re-bars    Three Ply 14,200        No      4.5    With re-bars    ______________________________________

                                      TABLE 2    __________________________________________________________________________    SAMPLE         HC COMPOSITE                    THICK  DEFLECTION                                   COMMENTS    __________________________________________________________________________    1     2"            4-2" flat                    .187" re-bar                           13,000  In skin    2    2  8-1" flat                    .187 re-bar                           13,400  In skin    3    2  8-1" flat                    .187 re-bar                           17,000  In skin            8-1" flat                    .187 re-bar    On sides    4    2  8-1.25" .1 re-bar                           13,500  In skin    5    2  1-12 × 12" FG cloth                           11,000  In Skin    6    2  2-12 × 12" FG cloth                           13,000  In skin    7    2  2-12 × 12" FG cloth                           16,200  In skin            8-1" flat re-bar       On sides    8    2  2-12 × 12" FG cloth                           13,000  In skin            60% Mg silicate        In honeycomb    __________________________________________________________________________

Referring again to FIG. 15, load-bearing structure 110 consists of twopartial or half structures 150,152 combined at a joining line 154,conveniently by fusion welding. The welding typically leaves a visiblebead. Referring again to reinforcing fibrous members 140, these membersmay be disposed at an angle to one another so as to form, in separatelayers as defined by partial structures 150,152, a lattice of membersintersecting at approximately right angles. Other patterns may be used;for instance, the members could have a parallel orientation. Also, ifdesired, members 140 could be omitted from one or both of structures150,152.

With reference to Table 2, comparative deflection data for load-bearingstructures similar to FIGS. 14 and 15 but lacking reinforcing members140 disposed as shown, are given. Each structure is 12"×12"×3 3/4",including about one-half inch, top skin and bottom skin sandwichinghoneycomb cellular structure, and a rubber surface made from choppedtires. "HC" is the height of the honeycomb cells, which are formed ofpolyethylene. The composite used is fiberglass/polyethylene re-bar, oris woven fiberglass fabric (0.010" thickness)/polyethylene composite.The composite is thermally bonded to adjacent upper and lowerpolyethylene skin layers. The structures are made by welding togethertwo half structures similar to structures 150,152. The deflection is theforce in PSI required to produce a one-half inch deflection across a teninch span.

In sample 1, 4 pieces of 2" wide, flat re-bar of 0.187" thickness and12" length are spaced apart to form a reinforcing fiber layer in eachmultilayer skin. Sample 2 is the same, except that 8 pieces of 1" wide,flat re-bar are used. Sample 3 is the same as sample 2, except that inaddition, 4 pieces of the 1" wide flat rebar are thermally bonded withinthe four side walls of the top half of the structure, and likewise 4pieces are thermally bonded within the four side walls of the bottomhalf of the structure. Sample 4 shows that, compared to sample 2, piecesof thinner but wider re-bar provide a structure of about equal strength.

Samples 5 and 6, which are closest to the structure of FIGS. 14 and 15,differ from one another in the number of layers of fiberglass cloth.Sample 6 surprisingly shows that a reinforcing fiber layer consisting ofa pair of vertically spaced apart, fiberglass webs imparts substantiallyequal structural strength compared to a reinforcing fiber layerconsisting of horizontally spaced apart, 0.187", flat re-bar (sample 1).

Sample 7 is the same as sample 6, except that as in sample 4, 8 piecesof the 1" wide flat rebar are thermally bonded within the four sidewalls of the top and bottom halves of the structure. Sample 8 is thesame as sample 6, except that the molding resin forming the honeycomb isloaded with 60 wt. % magnesium silicate, to increase the density, aslater explained.

Reinforcing fiber used in the present invention, is preferably highstrength, structural fiber. Exemplary high strength, structural fiberincludes glass fiber such as E glass and S glass, carbon fiber, aramidfiber, polyphenylene sulfide fiber, and liquid crystalline polymerfiber. Mixtures of fiber may be used.

Illustrative reinforcing fiber/matrix resin composites useful in thepresent invention, include fiberglass/polyethylene for use with apolyethylene molding resin, and fiberglass/polypropylene for use with apolypropylene molding resin. Fiberglass bar or fabric in the skin may becombined with fiberglass bar in the cell-forming walls.

In accordance with the invention, vertical walls 114 of the cellularcore structure may include bleeder holes 160 for connecting the cells.This feature allows the cellular structure to be filled with fluid forthose applications where load-bearing structure 110 will be layed on theground and may be at risk of floating away, if flooded. Typically, itwill be suitable to use water as the fluid, provided that the structurehas a specific gravity greater than one. A specific gravity greater thanone may be advantageously provided by use of a molding resin having adensity in the range of about 0.945 to 0.965 gm/cc, suitably about 0.950to 0.960 gm/cc, in combination with fiber such as that in fiber/matrixresin composite.

For applications where rough treatment or abuse may occur or leakage isotherwise of concern, a suitable amount of a conventional gelling agentmay be added to the fluid in an effective amount. Thus, water may begelled by, for instance, adding a suitable amount of crosslinkedpolyacrylamide mixed with the water.

In areas where freezing may be a problem and water is used as the fluid,an antifreeze agent such as ethylene glycol, alcohol or sodium chloridemay be added to the water in an effective amount. Alcohol/water mixturesmay be preferred for environmental reasons and have been found to workwell with polyacrylamide gelling agent with levels of alcohol up to 35to 50%.

A specific gravity greater than one may be provided by a suitableloading of a density-increasing, additive material in addition to fiber.To this end, a mineral filler may be added to the molding resin.Exemplary mineral fillers include talc and calcium carbonate. Asindicated in Table 2, suitable loadings of mineral filler may range upabout 60 wt. % or more to obtain the desired specific gravity, withoutdeleterious effect upon structural strength. Even though mineral fillermay provide a stiffening benefit, loss of beneficial properties mayoccur. As a result, use of a high density resin as previously described,is preferred.

Referring to FIG. 16, a stepped-down lip 170 is layered similar to skinlayer 116. Accordingly, lip 170 includes a gripping layer 172, and alayer 176 of reinforcing fiber/matrix resin composite disposed betweenand thermally bonded to thermoplastic polymer layers 174,178, forming aunitary multilayer structure. The gripping layer may be omitted ifdesired. Depending upon the load-bearing requirements, layer 176 may beconstituted differently from the reinforcing fiber/matrix resincomposite in the skin layer. As indicated in the Figure, layer 176 maybe flat composite re-bar similar to fibrous reinforcing member 140.

In accordance with the present invention, a method of molding isprovided as now illustrated with reference to loading-bearing structure110. First, a layer of chopped rubber from, for instance, choppedrecycled tires, is placed in a lower mold member to a suitable depth.The molding member has an ordinary smooth interior surface. Asmentioned, layer 138 of granular rubber may be omitted from structure110 depending upon the intended use of the resultant load-bearingstructure; thus, this first step is optional.

Thereafter, a layer of a suitable thermoplastic material is added tomake layer 134. Then, a reinforcing fiber layer comprising fiberpretreated so as to coated with a matrix resin, is added. Beneficially,as shown, the fiber layer may include a pair of spaced apart, fibrousweb structures encapsulated within matrix resin. As mentioned, thisfiber layer may be made by placing into the mold in sequence, the fivelayers earlier mentioned. Then, a layer of a suitable thermoplasticmaterial is added.

The thermoplastic materials are conveniently added in solid form aspellets, powder, granules and so forth, to the molding member to providelayers of an appropriate depth determined by the desired thickness ofthe corresponding resultant layer. In the case of the secondthermoplastic material, the depth needs to be adequate to make innerlayer 122 and also cell-forming walls 114.

As previously described, the thermoplastic materials are advantageouslycompatible with the matrix resin so that layers 122,126,134 thermallybond together into a unitary structure under the molding conditions. Thethermoplastic materials may be the same or different. Beneficially, thematrix resin has a higher melt index than the molding resin, and thisenables the matrix resin to flow better; however, the matrix resin andmolding resin may be the same type of thermoplastic, for instance,polyethylene or polypropylene.

In an upper mold member configured to form walls 114 of the cellularstructure, a plurality of fibrous reinforcing members 140 are verticallydisposed within wall-forming portions of the mold member. Then, theupper and lower mold members are combined and subjected to conventionalheat and pressure. To this end, the molds are heated to and maintainedat a temperature above the melt temperature of the highest meltingresin. This temperature will vary particularly depending upon the resinsused. Ordinary molding pressures are used, with the molding pressureused depending upon factors including, as earlier mentioned, the matrixresin selected.

After cooling, molded structure 152 is separated from the lower andupper mold members. The structure includes an inner layer 122 integralwith cell-forming walls 114 generally perpendicular thereto, andadvantageously includes a plurality of fibrous reinforcing members 140vertically molded into and thermally bonded to cell-forming walls 114.Beneficially, the cell-forming walls are provided with a vertical taperof about 2 degrees to assist demolding. This process is repeated to formmolded structure 154, and molded structure 152 and 154 are weldedtogether to make load-bearing structure 110.

Use of a cellular structure and reinforcing fiber/matrix resincomposites results in stiffness with less part weight, shorter coolingcycle during production and less problem with stress cracking, than ifthe structure were a solid structure, that is, without cellularstructure.

In the preceding description of the present invention, there are shownand essentially described only preferred embodiments of this invention,but as mentioned, it is to be understood that the invention is capableof changes or modifications within the scope of the inventive conceptexpressed herein. Several changes or modifications have been brieflymentioned for purposes of illustration.

We claim:
 1. A load-bearing structure comprisinga molded structurecomprising a first skin structure covering a face of a first pluralityof generally vertical walls forming a first plurality of shaped hollowswhich have an opposing face, and a molded structure comprising a secondskin structure covering a face of a second plurality of generallyvertical walls forming a second plurality of shaped hollows whichlikewise have an opposing face, joined together with the opposing facesin contact at a joining line formed by said first plurality of generallyvertical walls and by said second plurality of generally vertical walls,said opposing faces being otherwise uncovered, to form a cellular corederived from said first plurality of shaped hollows and from said secondplurality of shaped hollows, and sandwiched between and covered by saidfirst and second skin structures, wherein the walls forming saidcellular core include a plurality of cell-connecting apertures adjoiningsaid joining line.
 2. The load-bearing structure of claim 1, furtherincluding a gel formed from a fluid and an effective amount of asuitable fluid gelling agent.
 3. The load-bearing structure of claim 1,further comprising density-increasing, additive material in an amountsufficient to provide said structure with a specific gravity greaterthan one.
 4. The load-bearing structure of claim 1, wherein said firstskin structure is layered and comprises an innermost layer comprising athermoplastic resin, and said generally vertical walls covered by saidfirst skin structure, likewise comprise said thermoplastic resin.
 5. Theload-bearing structure of claim 1, wherein said first skin structurecomprises a reinforcing composite fusion bonded to an adjacentcompatible thermoplastic layer comprising a polyolefin selected from thegroup consisting of polyethylene and polypropylene, wherein saidcellular core comprises honeycomb-shaped cells, and wherein said secondskin structure comprises reinforcing fibers in the form of spaced apartfibrous webs.
 6. The load-bearing structure of claim 1, furthercomprising a reinforcing composite molded within said generally verticalwalls covered by said first skin structure, and a compatiblethermoplastic resin disposed adjacent to said reinforcing composite,wherein said reinforcing composite is fusion bonded to said compatiblethermoplastic resin.
 7. The load-bearing structure of claim 1, whereinsaid first skin structure comprises a traction-enhancing surface layerand an underlying thermoplastic layer.
 8. The load-bearing structure ofclaim 1, wherein said first skin structure comprises a rubber surfacelayer and an underlying thermoplastic layer.
 9. A load-bearing structurecomprising a lower stratum and an upper stratum each of which extendsbeyond the other to provide an upper offset and a lower offset, andbeing formed of a first molded structural member joined to an underlyingsecond molded structural member, wherein said lower offset comprises aportion of said second molded structural member and a first portion ofsaid first molded structural member that covers said portion of saidsecond molded structural member, and said upper offset comprises asecond portion of said first molded structural member and an additionalportion of said second molded structural member that underlies saidsecond portion.
 10. The load-bearing structure of claim 9, wherein saidportion of said second molded structural member comprises shaped hollowshaving an open face covered by said first portion of said first moldedstructural member.
 11. The load-bearing structure of claim 10, whereinsaid shaped hollows are honeycomb-shaped and comprise generally verticalwalls comprising a thermoplastic resin.
 12. The load-bearing structureof claim 9, further comprising a core of cells having generally verticalwalls disposed within said load bearing structure.
 13. The load-bearingstructure of claim 9, further comprising reinforcing composite and acompatible thermoplastic resin disposed adjacent to said reinforcingcomposite, wherein said reinforcing composite is fusion bonded to saidcompatible thermoplastic resin.
 14. The load-bearing structure of claim9, further comprising reinforcing fiber in the form of spaced apart,fibrous webs.
 15. The load-bearing structure of claim 9, wherein saidfirst molded structural member and said second molded structural memberare visually identical parts.
 16. The load-bearing structure of claim 9,wherein said first molded structural member is a mirror image of saidsecond molded structural member.
 17. The load-bearing structure of claim9, wherein said first portion defines a first plane and said secondportion defines a second plane, and wherein said portion of said secondmolded structural member and said additional portion of said secondmolded structural member likewise define different planes.